Optional Parameters in Go

Question

Can Go have optional parameters  Or can I just define two functions with the same name and a different number of arguments

User · Answer

For arbitrary  potentially large number of optional parameters  a nice idiom is to use Functional options   For your type Foobar  first write only one constructor   func NewFoobar options    func  Foobar  error    Foobar  error     fb     amp Foobar             write initializations with default values       for    op    range options      err    op fb      if err    nil         return nil  err             return fb  nil     where each option is a function which mutates the Foobar  Then provide convenient ways for your user to use or create standard options  for example    func OptionReadonlyFlag fb  Foobar  error     fb mutable   false   return nil    func OptionTemperature t Celsius  func  Foobar  error     return func fb  Foobar  error       fb temperature   t     return nil         Playground  For conciseness  you may give a name to the type of the options  Playground     type OptionFoobar func  Foobar  error   If you need mandatory parameters  add them as first arguments of the constructor before the variadic options   The main benefits of the Functional options idiom are     your API can grow over time without breaking existing code  because the constuctor signature stays the same when new options are needed  it enables the default use case to be its simplest  no arguments at all  it provides fine control over the initialization of complex values    This technique was coined by Rob Pike and also demonstrated by Dave Cheney

User · Answer

A nice way to achieve something like optional parameters is to use variadic args   The function actually receives a slice of whatever type you specify  func foo params    int        fmt Println len params      func main         foo       foo 1      foo 1 2 3

User · Answer

Another possibility would be to use a struct which with a field to indicate whether its valid  The null types from sql such as NullString are convenient  Its nice to not have to define your own type  but in case you need a custom data type you can always follow the same pattern  I think the optional-ness is clear from the function definition and there is minimal extra code or effort   As an example   func Foo bar string  baz sql NullString     if  baz Valid           baz String    defaultValue           the rest of the implementation

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You can encapsulate this quite nicely in a func similar to what is below   package main  import            bufio           fmt           os     func main             fmt Println prompt       func prompt params    string  string           prompt                 if len params   gt  0                   prompt   params 0                    reader    bufio NewReader os Stdin          fmt Print prompt          text       reader ReadString   n           return text     In this example  the prompt by default has a colon and a space in front of it                   however you can override that by supplying a parameter to the prompt function   prompt  Input here - gt       This will result in a prompt like below   Input here - gt

User · Answer

Neither optional parameters nor function overloading are supported in Go   Go does support a variable number of parameters  Passing arguments to     parameters

User · Answer

Go does not have optional parameters nor does it support method overloading      Method dispatch is simplified if it   doesn t need to do type matching as   well  Experience with other languages   told us that having a variety of   methods with the same name but   different signatures was occasionally   useful but that it could also be   confusing and fragile in practice    Matching only by name and requiring   consistency in the types was a major   simplifying decision in Go s type   system

User · Answer

You can use a struct which includes the parameters   type Params struct     a  b  c int    func doIt p Params  int     return p a   p b   p c        you can call it without specifying all parameters doIt Params a  1  c  9

User · Answer

No -- neither   Per the Go for C   programmers docs      Go does not support function   overloading and does not support user   defined operators    I can t find an equally clear statement that optional parameters are unsupported  but they are not supported either

User · Answer

You can pass arbitrary named parameters with a map  You will have to assert types with  quot aType   map key    foo type  quot  if the parameters have non-uniform types  type varArgs map string interface    func myFunc args varArgs         arg1     quot default quot      if val  ok    args  quot arg1 quot    ok           arg1   val  string             arg2    123     if val  ok    args  quot arg2 quot    ok           arg2   val  int             fmt Println arg1  arg2     func Test test         myFunc varArgs  quot arg1 quot    quot value quot    quot arg2 quot   1234

User · Answer

Go language does not support method overloading  but you can use variadic args just like optional parameters  also you can use interface   as parameter but it is not a good choice

User · Answer

I ended up using a combination of a structure of params and variadic args   This way  I didn t have to change the existing interface which was consumed by several services and my service was able to pass additional params as needed   Sample code in golang playground  https   play golang org p G668FA97Nu

User · Answer

I am a little late  but if you like fluent interface you might design your setters for chained calls like this   type myType struct     s string   a  b int    func New s string  err  error   myType     if s              err   errors New         Mandatory argument  s  must not be empty          return  amp myType s  s     func  this  myType  setA  a int  err  error   myType     if  err    nil       if a    42          err   errors New  42 is not the answer          else         this a   a             return this    func  this  myType  setB  b int     error   myType     this b   b   return this     And then call it like this   func main       var err error   nil   instance        New  hello    amp err       setA 1   amp err       setB 2   amp err     if err    nil       fmt Println  Failed     err      else       fmt Println instance          This is similar to the Functional options idiom presented on  Ripounet answer and enjoys the same benefits but has some drawbacks    If an error occurs it will not abort immediately  thus  it would be slightly less efficient if you expect your constructor to report errors often  You ll have to spend a line declaring an err variable and zeroing it    There is  however  a possible small advantage  this type of function calls should be easier for the compiler to inline but I am really not a specialist

[go] Optional Parameters in Go?

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